1. Field of the Invention
The present invention relates to a process for producing xylooligosaccharide from a lignocellulose pulp.
2. Description of the Related Art
It is known that oligosaccharides are useful as a saccharide material for lactic acid bacteria-containing beverages and chocolate-containing food which are classified as specific healthful foods having an effect of promoting the selective propagation of lactic acid bacteria and of contributing to keeping the stomach and intestines in good condition, and are utilized as emulsifying agents and skin-moisturizing agents for drugs and sanitary materials. Also, the oligosaccharides are used as additives not only for foods for human beings but also for feed for livestock.
Generally, almost all of the oligosaccharides used in the specific healthful goods have an intestine-controlling activity for decreasing the colon bacteria which are undesirable bacteria in the intestines and clostridium bacteria which are putrefaction fermentation bacteria in the intestines and on the contrary for increasing bifid bacteria which are known as desirable bacteria in the intestines. For example, the mold bran of wheat are polysaccharides comprising hemicellulose having, as a backbone drum, xylane groups, are scont-decomposible vegetable fibers, and are used as an additive for food having intestine-controlling activity.
The intestine-controlling activity of the wheat mold bran is assumed to be derived from xylooligosaccharides produced by decomposing the wheat mold bran by the intestinal bacteria in the intestine. Also, it is assumed that the xylooligosaccharides derived from the wheat mold bran promotes the selective increase of the bifid bacteria which are desirable bacteria in the intestines, and also causes the colon bacteria which are undesirable bacteria in the intestines to be decreased. The colon bacteria and the putrefaction fermentation bacteria in the intestines are known to produce carcinogenic substance which then are increased in the intestines, and thus to keep good health over a long period it is important that the numbers of the colon bacteria and the putrefaction fermentation bacteria are decreased in the intestine.
It is assumed that the longer the chain length of the xylooligosaccharides, the higher the promotion effect on the selective propagation of the xylooligosaccharides ingested by the human body. Particularly, the xylooligosaccharides in the form of tri-or more-mers contribute the selective propagation of the bacteria.
The xylooligosaccharides available in trade at the present time, are produced from a material made from herbages, for example, wheat mold bran or corn-cob. In the material made from the herbages, the xylan backborn chain has branched side chains made from saccharide other than xylan, for example, glucuronic acid. When an oligosaccharide consisting of only the xylan is produced from the xylan having many side chains, only oligosaccharide having a relatively low degree of polymerization can be produced. At the present, in almost all of the xylooligosaccharides now in trade, the oligosaccharides from which the xylooligosaccharides are formed are in the form of dimers. Accordingly, the xylooligosaccharides having a higher degree of polymerization than that the dimer are strongly demanded.
The xylooligosaccharide is produced from xylan which is one of the principal components for forming plants. As a xylan in the form of a straight chain and consisting of xylose only, stalks of esparto and tabacco are known. As xylan in the form applicable for industry, arabinoxylan contained in wheat mold bran and corn cob which are produced as a by-product in the corn production, glucuronoarabinoxylan contained in softwoods and glucuronoxylan contained in hardwoods are known. In the saccharides contained in these xylans applicable for industry, arabinose, glucuronic acid, 4-O-methyl glucuronic acid, glucose and galactose are contained in addition to the xylose. The proportions of the xylose and the other saccharides are variable depending on the type of the plants.
Japanese Patent No. 146,374 discloses a method of producing xylan in which bagasse and other grasses, leguminous plants, and linaceace plants which contain pentosan in a high content are digested in the presence of an organic acid such as acetic acid under high pressure, to make the scant water-soluble protosan contained in the starting material soluble in water and to make the tissues other than fibrovascular bundles weak and brittle; the digested material is ground and washed; and the resultant fiber bundle is subjected to a known pulping procedure by, for example, soda method, alkali sulfite method or sulfate salt method, to separate and collect pentosan from the pulped material.
The xylose is contained in a high content in wood, and the content of xylose based on the total weight of the wood is about 6 to 10% in softwood, and about 20% in hardwood, and thus the xylose is an important component of the wood. (Migita Nobuhiko et al. “Wood Chemistry” published by Kyoritsu Shuppan, page 73 (1968)). It is known that xylan is extracted from wood and is used to produce xylooligosaccharide, xylose, and xylitol, in practice.
At the present time, the pulp is produced mainly from wood chips by chemical treatment or mechanical treatment. When the pulp is produced and collected from the wood. Lignocellulose material, particularly the residual component of the wood chips after the pulp comprising cellulose collected from the wood chips mainly comprises lignin and hemicellulose which are contained in the waste liquid from the pulp-producing procedure.
Various technologies of isolating specific components from the waste liquid of the pulp-producing procedure and utilizing the isolated components for woods or food additives have been used in practice. In an old technology, vanillin had been produced by oxidizing a waste liquid from a sulfite pulp-producing procedure with air or oxygen in an alkaline reaction system at a temperature of about 160° C.
Also, Japanese Examined Patent Publication No. 43-731 discloses a method of producing xylose from hemicellulose contained in a waste liquid delivered from a pulping procedure by a pre-hydrolysis method in a kraft pulp-production.
Also, production of a seasoning matter has been practically carried out by preparing yeast by using, as a culture medium, saccharide contained in a large amount in the waste liquid delivered from the sulfite pulp producing procedure, and collecting the seasoning matter such as nucleic acid from the yeast per se or yeast-containing composition. Further, Japanese Unexamined Patent Publication No. 51-101,193 discloses a method of producing a protein from the waste liquid discharged from the sulfite pulp-producing procedure, and Japanese Unexamined Patent Publication No. 56-144,742 discloses a method of producing ethyl alcohol from the waste liquid from the sulfite pulp-producing procedure.
L. Viikari et al., Biotechnol, Pulp Paper Ind. (Stockholm) pp 67 to 69 (1986) reports when a pulp is treated with xylanase and the whiteness of the pulp is improved by this treatment. Also, Mora et al. report, in F. Mora et al., J. Wood Chem. Technol., Vol. 6, pp 147-165 (1986), that the mechanical strength of pulp can be enhanced by treating the pulp with xylanase. This report further discloses that a filtrate of a reaction mixture produced by treating a kraft pulp of birch wood with xylanase contains, xylose and xylooligosaccharides including di- to octa-mers of xylose. Further, D. J. Senior et al., Biotechol, Lett., vol. 10, pp 907-912 (1922) discloses that a filtrate obtained from a reaction mixture prepared by treating a kraft pulp of aspen wood with xylanase contains xylose and xylooligosaccharides. However, the above-mentioned reports are quite silent as to a recovery of xylooligosaccharides from a filtrate of an enzyme treatment reaction mixture.
As a method of producing xylooligosacccharides, U.S. Pat. No. 4,181,796 (corresponding to Japanese Unexamined Patent Publication No. 53-35,005) discloses a method in which a botanical material is treated, together with acetic acid, with saturated steam at a temperature of from 160° C. to 230° C. under pressure, and water-extractable xylan and xylan fragments are separated from monosaccharide and other impurities. In accordance with this method, the xylan and xylan fragments can be refined by a treatment with an OH-type strong basic ion-exchange resin and by an ultra-filtration with a high efficiency.
Japanese Unexamined Patent Publication No. 61-242,592 discloses a biochemical method in which xylan is treated with xylanase produced by microorganism in Bacillus group, and xylooligosaccharides are produced from a filtrate prepared from the reaction mixture of the xylanase treatment, by collecting a clear filtrate from the reaction mixture after the xylanase is heat-deactivated, and concentrating the clear filtrate to provide a syrup of xylooligosaccharide, and optionally freeze-drying the syrup to provide a powder of xylooligosaccharide
Also, according to Japanese Unexamined Patent Publication No. 63-112,979, in a method of recovering xylooligosaccharide from a filtrate of a reaction mixture prepared by treating hardwood xylan with xylanase derived from Trichoderma, the filtrate is decolored by activated carbon, the activated carbon is removed from the filtrate by using a filter press, the saccharide absorbed in the activated carbon is recovered by using a 15% ethanol, the recovered saccharide is treated with an ion-exchange resins (trademark: AMBERLITE IR-120B and AMBERLITE IR-410, to remove salts, and then is concentrated by a reverse osmosis membrane to obtain xylooligosaccharide containing xylobiose in a high content.
These publications are, however, quite silent as to the recovery and refining of xylooligosaccharides from a filtrate prepared from a reaction mixture in which a chemical pulp is treated with hemicellulase.
When the xylooligosaccharide contained in the filtrate of the reaction mixture in which the pulp is treated with hemicellulase, is recovered and refined by the method disclosed in Japanese Unexamined Patent Publication No. 63-112,979, the necessary cost is too high and thus this method is not utilized in practice. The reasons for the uselessness are in that the waste liquid delivered from the enzyme-treatment procedure for the pulp is in too large a volume, and contains the saccharide in a low content, and the content of impurities, for example, various organic acids generated during the pulping and oxygen-bleaching procedures for lignin, cellulose and hemicellulose, in the filtrate is very high. Namely, in this case, the activated carbon and the ion-exchange resins must be employed in a large amount for the recovery and refining; the concentration procedure of the filtrate by the reverse osmosis membrane causes the waste liquid to be generated in a large amount; the waste liquid contains water-insoluble components, for example, lignin, in a high content; and thus a large scale of production apparatus is necessary for the method of the Japanese publication.
In the conventional process for producing xylooligosaccharide, generally, arabinoxylan which is contained in wheat mold bran and corn cob obtained, as a by-product, from the production of corn foods, and glucuronoxylan of hardwood, are employed, as starting xylan materials applicable in industry. These materials are extracted by the above-mentioned method, and extract is treated with hemicellulase such as xylanase, to produce xylooligosaccharide comprising mono- to deca-mers of xylase, preferably mono-to penta-mers of xylose. The xylan containing material applicable for industry contains, in addition to xylose, arabinose, glucuronic acid, 4-O-methylglucuronic acid, glucose and glactose, and other mono-saccharides (as disclosed in, for example, Japanese Unexamined Patent Publication No. 4-53,801). To obtain xylooligosaccharide consisting of pure xylose only, the resultant xylooligosaccharide must be further refined in an accurate manner. Thus, a low cost process for producing the xylooligosaccharide is strongly demanded.
It is known that the xylanase treatment applied to the kraft pulp enables the necessary amount of bleaching chemicals for the bleaching process for the pulp with the bleaching chemical to be reduced. In the xylanase treatment, since the xylan contained in the pulp is hydrolyzed with xylanase, the resultant waste water discharged from the bleaching system contains xylose and xylooligosaccharide separated from the pulp in large amount. In paper industry, to reduce the amount of process water used, an amount of water used in a step of the bleaching procedure is returned to and utilized in another step before the above-mentioned step. Therefore, the water used in a step before the enzyme treatment step contains xylan-decomposition products, for example, xylose and xylooligosaccharide, isolated by xylanase.
The above-mentioned xylose and xylooligosaccharide have reducing terminal groups, for example, aldehyde groups, the reducing terminal groups are oxidized in the oxidation-bleaching procedure, for example, an oxygen-bleaching procedure and the xylose and xylooligosaccharide are converted to carboxylic acids and further to oxidized furan derivatives and then to colored furan condensation products, to consume the bleaching chemicals. Thus, in this case, the bleaching agents consumed due to the presence of the saccharides must be supplemented. Also, in the oxygen bleaching procedure under a high alkaline condition, the aldehyde groups are oxidized and the resultant carboxylic acid causes the pH value of the bleaching system to be reduced. Thus the pH values of the bleaching system must be controlled to a desired level by increasing the amount of alkali to be added to the bleaching system to compensate the reduction in pH.
In an attempted method in which xylose and xylooligosaccharide produced by the xylanase treatment is not returned to a preceeding bleaching step, the reducing saccharides are removed from the waste water discharged from the enzyme treatment system, and the resultant saccharide-free waste water is returned to a preceeding bleaching step. However, the waste water from the pulp production is generated in a large amount, and thus the removal of the saccharide by a conventional method, for example, the reverse osmosis membrane method, causes a very large scale of apparatus to be provided. Therefore, the above-mentioned removal of saccharide has not yet been carried out at a low cost.